1. Field of the Invention
The present invention relates to photoelectrophoretic photography and, particularly, to an improved process for producing images by utilizing a photoelectrophoretic photographic technique in which an injection electrode having an electrically insulating layer is used.
2. Description of the Prior Art
The photoelectrophoretic photographic process is well-known, e.g., as described in U.S. Pat. No. 3,384,565, U.S. Pat. No. 3,510,419 (corresponding to Japanese Patent Publication No. 21781/68), etc. and comprises the following steps of: (1) sandwiching an electrically insulating liquid containing light-sensitive, electrically charged particles (hereinafter referred to as an "ink") between a pair of electrodes at least one of which is partially transparent to light and (2) image-wise exposing the ink to light through the transparent electrode while simultaneously applying an electrical potential across both electrodes to cause the above-described charged particles to migrate in an image-wise pattern, wherein either or both of the particles remaining on the above-described transparent electrode through which they were image-wise exposed or/and the particles held firmly on the opposing electrode to the above-described transparent electrode through migration produce image(s). Such a process is described in detail in, for example, U.S. Pat. No. 3,510,419 (corresponding to Japanese Patent Publication No. 21781/68).
In photoelectrophoretic photography as described above, one of the pair of electrodes is called an injection electrode and the other of the electrodes is called a blocking electrode. The injection electrode corresponds to the electrode to which the charged particles present in the ink adhere upon exposure to the action of an electric field in the dark, which electrode has a polarity opposite that of the electric charge which the charged particles possess in the dark. On the other hand, the blocking electrode corresponds to the electrode toward which the optically-exposed particles migrate due to the influence of an electric field to be held thereon. This blocking electrode has the same polarity as that of the electric charge which the charged particles possess in the dark.
In many ordinary cases, the injection electrode has an electrically conductive surface, while the blocking electrode has an electrically insulating surface. However, electrodes where this situation is reversed may be also employed. Generally speaking, in conventional cases, these electrodes may possess any electrical properties at the surfaces thereof. Such being the case, only in particular situations are the electrical properties to be possessed by the surface of each electrode specified in advance. For instance, it is desirable for the injection electrode to have an electrically insulating surface when the blocking electrode has an electrically conductive surface.
An improved process in the art of photoelectrophoretic photography is disclosed in, for example, British Pat. No. 1,331,622, where an attempt was made to discover some way for causing all of the charged particles in the ink to have the same polarity and to control the deterioration in image quality resulting from a flow of the ink in the image-forming process. More specifically, in this process an image-wise exposure to light through a transparent electrode and an application of a voltage of a proper magnitude across a pair of electrodes are carried out simultaneously to thereby cause an image-wise distribution in the migration of charged particles. In this improved process corona ions having the same polarity as the polarity present at the blocking electrode due to the source of electric potential applied across the pair of electrodes in the subsequent image-forming process are showered on the ink coated on the surface of the injection electrode and, then, the resulting ink is sandwiched between the injection and the blocking electrodes, and exposed simultaneously to the action of an electric field and radiant energy in an image-wise pattern.
However, this process has proved to be effective only for the above-described purposes, and has been found to possess the following disadvantage in addition. Where the surface of the injection electrode is electrically insulating, corona ions of the same polarity as the polarity of the blocking electrode, to which an electric potential is to be applied in the subsequent image-forming process, accumulate on the electrically insulating surface of the injection electrode causing the electrically insulating surface to become charged and consequently, an electric field created by corona ions accumulating on the electrically insulating surface greatly weakens the effectiveness of the electric field to be applied to the charged particles incorporated in the ink in the subsequent image-forming process.
Accordingly, even though the charged particles in the ink are activated optically, an image can not be formed or only a poor image can be formed in this process as described above, because it is difficult to apply an electric field of the strength necessary to force the charged particles to migrate through the ink.
In order to eliminate the above-described defect and to thereby produce images of good quality, a powerful source of electric potential which can drown the electric field created by the corona ions accumulating on the electrically insulating surface and, further, which can generate in the particles an electric field sufficiently high that the optically-activated particles are forced to migrate must be connected between both the injection and the blocking electrodes.
Application of an electric field of such a high level across the injection and the blocking electrodes requires the space between the injection electrode and the blocking electrode to be insulated, and the insulation of a material to be grounded must be good enough to withstand the electric field applied. Therefore, a large-size apparatus is required for achieving such a high degree of insulation, which is costly. In addition, air tends to be ionized by sparks generated upon separating one electrode from the other electrode after the image-formation, which tends to cause a deterioration in the quality of images.